System and method for fabricating a stimulation lead

ABSTRACT

In one embodiment, a system for wrapping biomedical conductor wires about core material, comprises: a payout assembly and a take-up assembly for controllably paying out the core material and taking up the core material with the wrapped conductor wires; a turntable; a plurality of carriers, disposed on the turntable, for letting out the conductor wires; and a die for applying force to the conductor wires as the wires are wrapped about the core material, the die adapted to rotate according to group rotation of the plurality of carriers by the turntable during operation of the system, wherein the die comprises one or more features asymmetrically arranged about a circumference of the die, the one or more features adapted to direct the conductor wires from the plurality of carriers onto the core material in an axially repeating pattern of groups of closely spaced wires with each group separated by a distance larger than the spacing between adjacent wires within each group.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/247,264, filed Sep. 30, 2009, which is incorporated herein byreference.

TECHNICAL FIELD

This application is generally related to a system for wrapping conductorwires about core material for fabrication of a stimulation lead and amethod of fabricating a stimulation lead for stimulation of tissue of apatient.

BACKGROUND

Neurostimulation systems are devices that generate electrical pulses anddeliver the pulses to nerve tissue to treat a variety of disorders.Spinal cord stimulation (SCS) is the most common type ofneurostimulation. In SCS, electrical pulses are delivered to nervetissue in the spine typically for the purpose of chronic pain control.Applying electrical energy to the spinal cord associated with regions ofthe body afflicted with chronic pain can induce “paresthesia” (asubjective sensation of numbness or tingling) in the afflicted bodilyregions which can effectively mask the transmission of non-acute painsensations to the brain.

Neurostimulation systems generally include a pulse generator and one ormore leads. The pulse generator is typically implemented using ametallic housing that encloses circuitry for generating the electricalpulses, control circuitry, communication circuitry, a rechargeablebattery, etc. The pulse generating circuitry is coupled to one or morestimulation leads through electrical connections provided in a “header”of the pulse generator.

Each stimulation lead includes a lead body of insulative material thatencloses wire conductors. The distal end of the stimulation leadincludes multiple electrodes that are electrically coupled to the wireconductors. The proximal end of the lead body includes multipleterminals, which are also electrically coupled to the wire conductors,that are adapted to receive electrical pulses. The distal end of arespective stimulation lead is implanted at the location adjacent orwithin the tissue to be electrically stimulated. The proximal end of thestimulation lead is connected to the header to the pulse generator or toan intermediate “extension” lead.

The manufacture of stimulation leads is a relatively complex process.Some manufacturing techniques involve wrapping conductor wires withinsulative coatings about a mandrel in a helically manner. An example ofa system adapted to perform such winding is shown in U.S. Pat. No.7,287,366, entitled “Method for producing a multielectrode lead,” whichis incorporated herein by reference. The system described in the '366patent draws a mandrel through wire wrapping structure. As the mandrelis drawn into a spool, conductor wires are let out in controlled mannerby a plurality of “payout carriers.” The plurality of payout carriersare rotated as a group about the mandrel. Also, each payout carrier isrotated independently about its own axis to compensate for twistimparted by the group rotation to minimize the amount of residual forceleft on the wound wires. The final product in the '366 patent is aproduct with multiple conductor wires wound about the mandrel in helicalmanner. This product is then cut into separate lengths for fabricationof stimulation leads including attachment of electrodes and terminals.

Also, in known wire wrapping systems, force is applied to the wires asthe wires are served onto the mandrel to permanently deform or “preform”the wires to maintain the wires around the mandrel when the windingtension is released. The application of force may be implemented using a“winding die.” In operation, the individual wires pass over one or moreradii of a circular or toroidal die where the assembly of the wires andthe mandrel pass through a center hole of the die.

SUMMARY

In one embodiment, a system for wrapping biomedical conductor wiresabout core material, comprises: a payout assembly and a take-up assemblyfor controllably paying out the core material and taking up the corematerial with the wrapped conductor wires; a turntable; a plurality ofcarriers, disposed on the turntable, for letting out the conductorwires; and a die for applying force to the conductor wires as the wiresare wrapped about the core material, the die adapted to rotate accordingto group rotation of the plurality of carriers by the turntable duringoperation of the system, wherein the die comprises one or more featuresasymmetrically arranged about a circumference of the die, the one ormore features adapted to direct the conductor wires from the pluralityof carriers onto the core material in an axially repeating pattern ofgroups of closely spaced wires with each group separated by a distancelarger than the spacing between adjacent wires within each group.

The foregoing has outlined rather broadly certain features and/ortechnical advantages in order that the detailed description that followsmay be better understood. Additional features and/or advantages will bedescribed hereinafter which form the subject of the claims. It should beappreciated by those skilled in the art that the conception and specificembodiment disclosed may be readily utilized as a basis for modifying ordesigning other structures for carrying out the same purposes. It shouldalso be realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the appendedclaims. The novel features, both as to organization and method ofoperation, together with further objects and advantages will be betterunderstood from the following description when considered in connectionwith the accompanying figures. It is to be expressly understood,however, that each of the figures is provided for the purpose ofillustration and description only and is not intended as a definition ofthe limits of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a process for fabricating a lead body according to onerepresentative embodiment.

FIG. 2 depicts a mandrel for use in fabricating a lead body.

FIG. 3 depicts a segment of a lead body fabricated according to onerepresentative embodiment.

FIG. 4 depicts a wire wrapping system according to one representativeembodiment.

FIG. 5 depicts a portion of the system shown in FIG. 4 according to onerepresentative embodiment.

FIG. 6 depicts a set of gears for use in the system of FIG. 4 accordingto one representative embodiment.

FIGS. 7-9 depict a plurality of payout carriers for use in the system ofFIG. 4 according to one representative embodiment.

FIG. 10 depicts another view of a wire wrapping system according to onerepresentative embodiment.

FIG. 11 depicts a die for use in a wire wrapping system according to onerepresentative embodiment.

FIG. 12 depicts another die for use in a wire wrapping system accordingto one representative embodiment.

FIG. 13 depicts another die for use in a wire wrapping system accordingto one representative embodiment.

FIG. 14 depicts another die for use in a wire wrapping system accordingto one representative embodiment.

FIG. 15 depicts a cross-sectional view of lead body assembly 1500according to one representative embodiment.

FIG. 16 depicts a lead body fabricated according to one representativeembodiment.

FIG. 17 depicts a stimulation lead fabricated according to onerepresentative embodiment.

DETAILED DESCRIPTION

In one embodiment, a process for fabricating lead body material forstimulation leads begins with a continuous core material 10 shown inFIG. 1. In one embodiment, the core material 10 is apolytetrafluoroethylene (PTFE) coated stainless steel mandrel wire 12(shown in FIG. 2), although additional insulative layers may also beprovided according to other embodiments. Referring again to FIG. 1, thecore material 10 is then helically wrapped with a set of insulated wires14 at a wire wrapping system 15. Each of the wires 14 may include one ormore layers of insulation. In one embodiment, each wire 14 comprises aninner thin layer of perfluoroalkoxy (PFA) and outer thicker layer of athermoplastic silicone polycarbonate urethane (e.g., CARBOSIL™). Whileeight insulated wires are used in one embodiment, those skilled in theart will recognize that any suitable number of wires may be wrapped ontomandrel 12 according to other embodiments. In other embodiments,additional layers of wires 14 may be wound over the initial layer(s) ofwires.

In one preferred embodiment, wires 14 are wrapped about core material 10in an axially repeating pattern of groups 301 of closely spaced wireswith each group 301 separated by distance 302 that is larger than thespacing between adjacent wires within each group (FIG. 3). The distancebetween groups in FIG. 3 is by way of example and any suitable distancemay be employed according to some embodiments.

Referring again to FIG. 1, core material 10, now comprising mandrel 12and helically wrapped insulated wires 14 may now be spooled and laterunspooled (not shown) or fed directly to the next step in the process.In this next step, core material 10 may be selectively and repeatedlyheated in a reflow oven 18. The wires 14 are heated to a temperaturethat causes the insulation 16 of insulated wires 14 to approach orachieve a phase change, thereby becoming soft and adherent andultimately fusing together, by heating, melting and re-solidifying afterremoval from reflow oven 18.

At this point, the core material 10, now comprising mandrel 12 havinginsulated wires 14 at least partially fused about it, may now be spooled(step 19) onto a spool and stored for later work. Alternatively, step 19is not performed and core material 10 proceeds directly to the remainingsteps. Continuous core material 10 is cut (step 24) into individual leadbodies 21. Each individual lead body 21 may have a length of from about10 cm (4 in) to about 150 cm (60 in).

After the lead bodies 21 have been cut to length, mandrel 12 must beremoved from within in a mandrel removal step 28. This task may befacilitated by a coating of mandrel 12 that will ease removal, such as aPTFE coating. The mandrel removal step 28 may be a simple hand operationby a human worker.

Next, in an electrode creation step 30, electrodes and terminals areprovided on the distal and proximal ends of the lead body, respectively.Any suitable technique or process may be employed to provide theelectrodes and terminals. Exemplary electrode and terminal fabricationprocesses are described in U.S. Pat. No. 6,216,045, entitled“Implantable lead and method of manufacture,” and U.S. Pat. No.7,039,470, entitled “Medical lead and method for medical leadmanufacture,” which are incorporated herein by reference. Also, the leadbody could alternatively be connected to a paddle structure which holdselectrodes in a planar arrangement as is well known in the art.

Wire wrapping system 15 is shown in greater detail in FIG. 4. Portionsof wire wrapping system 15 are omitted from FIG. 4 for the sake ofclarity. For example, only two wires are shown in FIG. 4 being wrappedabout mandrel 12 and only the corresponding structures for these twowires are shown in FIG. 4. It shall be appreciated that the depictedstructures may be duplicated in wire wrapping system 15 in a givenimplementation according to any suitable number of wires selected to bewrapped about mandrel 12.

The wire wrapping process begins with mandrel payout assembly 80 andcore material take up assembly 86 that together maintain core material10 in well regulated motion and tension along its path. Simultaneously,controls and displays assembly 88 controls a power and linkage assembly82, which powers a wire payout assembly 84. Although one preferredembodiment permits the use of a keyboard for user input of controlparameters, as indicated in FIG. 4, an alternative embodiment provides asimple set of manual controls, such as knobs, for controls and displayassembly 88.

Assembly 84 includes turntable 114 upon which a set of payout carriers112 are supported. Wire wrapping system 15 is configured to permit avariable degree of back twist compensation, which is implemented byrotating carriers 112 relative to turntable 114 at an operator specifiedrate. In one embodiment, an operator manipulates controls and displayassembly 88 to place the right amount of back twist compensation ontowires 14. In an alternative embodiment, the operator enters the wire andmandrel dimensions and the pitch at which the wires are to be wrappedand control and display assembly 88 computes the degree of back twistcompensation necessary to prevent residual stress being placed ontowires 14.

Avoiding the placement of residual stress on wires 14 is important sothat this stress does not cause the wires to move spontaneously later inthe process, causing a deformation in the final shape of the lead body10, or inconsistent wire locations. After wrapping is complete, wrappedmandrel is spooled by core material take up assembly 86, which maintainsa constant tension to avoid deforming the core material 10. In analternative preferred embodiment, core material 10 is not spooled butprogresses immediately to the next stage of processing (e.g., reflow andfusing of the insulative coating material about wires 14).

In greater detail, the progress of core material 10 is maintained by thepayout assembly 80 and the take up assembly 86. Payout assembly 80includes a mandrel payout spool 100, a payout motor 102, and a dancerarm tension measurement device (not shown). Motor 102 is responsivesolely to the tension measurement, thereby maintaining constant tensionon core material 10. In take up assembly 86, core material take up spool105 is also motor driven (not shown) and solely responsive to tensionmeasurement dancer arm 103. Take up spool 105 is moved cyclically intoand out of the plane of FIG. 4, thereby causing core material 10 tospool in a repeated pattern. The tension placed on core material 10 canbe changed by changing the weighting on either dancer arm 103 or thedancer arm of payout assembly 80.

An additional portion of take up assembly 86 is the capstan 106, whichincludes an equal-diameter pair of wheels 108 and 110, about which corematerial 10 is looped several times. Each wheel 108 and 110 bearsseveral grooves along its exterior rim, to permit this looping whilepreventing the core material 10 from ever rubbing against itself.Capstan 106 is driven by an electric motor (not shown) and serves thefunction of stabilizing core material 10 as it is drawn through thesystem.

As shown in FIG. 4, core material 10 passes through the center of die900. Wires 14 pass around one or more radii of die 900. Preferably, die900 applies force to wires 14 to deform wires 14 for wrapping about corematerial 10. As shown in FIG. 4, die 900 is held by support struts 902which are, in turn, coupled to support columns 901. Support columns 901are mounted on platform 903. In one embodiment, platform 903 ismechanically coupled to a drive shaft that is also coupled to turntable114. Accordingly, platform 903 and die 900 rotate at the same rate asturntable 114. In alternative embodiments, die 900 is not mechanicallycoupled to turntable 114, but is independently driven to rotate the samerate as turntable 114. An isometric view of die 900 with support struts902, support columns 901, platform 903, and turntable 114 is shown inFIG. 10.

Die 900 is asymmetrically designed so that die 900 causes wires 14 to bewrapped about core material 10 in an axially repeating pattern of groupsof closely spaced wires with each group separated by a distance largerthan the spacing between adjacent wires within each group (see FIG. 3).

In some embodiments, wire wrapping system 15 controls the wire wrappitch using the ratio between the capstan 106 rotation rate and theturntable 114 rotation rate 96 (which equals the rotation rate of aturntable drive motor 132 (FIG. 5)) may be set prior to beginning a wirewrapping run. Likewise the backtwist compensation ratio 96, which is theratio of a payout carrier drive motor 134 rate (FIG. 5) to the turntabledrive motor 132 rate, may be set at the same time. Then, duringoperation, the speed of the entire process may be changed by changingthe turntable rotation rate command, which changes the capstan 106 turnrate and payout carrier drive motor 134 rate, automatically. In otherwords, during operation, the capstan 106 drive and the payout carrierdrive motor 134 are slaved to the turntable drive motor 132. The rate ofcapstan 106 effectively controls the turn rate of take up spool 105(FIG. 4) and pay out spool 100 (FIG. 4).

Referring to FIGS. 5 and 6, power and linkage assembly 82 includes aninner shaft 122 which drives the turntable 114, and an outer shaft 124which drives the payout carriers 112, by way of a system of gears 126.Inner shaft and outer shaft are driven by a first pulley 128 and asecond pulley 130, respectively. Each of these pulleys 128, 130 aredriven by belts 129 and 131 respectively that are in turn driven by theturntable motor 132 and the payout carrier motor 134, respectively.

The two motors 132 and 134 are managed by the control assembly 88 (FIG.4), which regulates their relative speed within a range of relativespeeds. As noted previously, the turn rate ratio of these two motors isset before a production run is begun. In one preferred embodiment thisrange extends from equal speed (payout carriers 112 stationary relativeto the turntable 114) to the case where the outer shaft rotates at onehalf the speed of the inner shaft (payout carriers 112 stationaryrelative to an absolute frame of reference).

A slip ring 140 (shown in FIG. 5) permits electric power to betransmitted to the rotating inner assembly that includes shafts 122 and124. On turn table 114, each payout carrier 112 includes a slip ring 142near its base for supplying electricity to the payout carrier 112. Eachpayout carrier 112 includes an electric wire tension control assembly144 that maintains a constant tension on the insulated wire 14 that isbeing threaded onto core material 10. Bearing assemblies 150, 152, 154and 156 facilitate the rotation of shafts 122 and 124. Plates 160 and162 support power and linkage assembly 82. A spider 164 supports a wireguide wheel 166 for each payout carrier 112, to further restrain thewires 14 as turntable 114 rotates.

Referring to FIGS. 7-9, each electric wire tension control assembly 144includes an electric motor 170 that drives a spool 172, both of whichare mounted on a payout carrier frame 173. Only four assemblies 144 aredepicted in FIG. 7 for the sake of clarity. Any suitable number ofassemblies 144 may be included according to some embodiments. Arespective wire 14 follows a path defined by a dancer arm 174 which isrotatably mounted by way of an axle 175 to frame 173. Dancer arm 174 hasa first dancer arm guide wheel 176 and a second dancer arm guide wheel178 about which wire 14 is threaded in an “S pattern.” Wire 14 proceedsabout a frame guide wheel 180 and through a payout carrier exit guide182. A dancer arm position measurement unit 184 monitors the position ofarm 174 and sends this information to an electric motor controller 186.Controller 186 commands the rate at which electric motor 170 turns. Thisarrangement permits control of the tension in wire 14 to an accuracy ofabout +1%.

FIG. 11 depicts die 1101 for wire wrapping system 15 according to onerepresentative embodiment. As shown in FIG. 11, die 1101 comprises aplurality of features (only one feature 1102 is annotated for the sakeof clarity) asymmetrically arranged about a circumference of an innersurface of die 1101. In this embodiment, the plurality of features 1102define a plurality of inner surfaces which form an array of planes thatintersect at the central axis of the die. In this embodiment, thefeatures 1102 are equally spaced about a limited arc of thecircumference of the inner surface leaving gap 1103 along thecircumference. In this embodiment, the features 1102 are equally spacedabout the limited arc (about 315° for eight features 1102), althoughother spacings (even or uneven) could be employed. Gap 1103 controls thespacing between respective groups of wires 14 applied to the corematerial 10.

FIG. 12 depicts die 1201 for wire wrapping system 15 according to onerepresentative embodiment. As shown in FIG. 12, die 1201 also comprisesa plurality of features asymmetrically arranged about a circumference ofan inner surface of die 1201. In lieu of the features having innersurfaces, die 1201 comprises a plurality of holes 1202 disposed over alimited arc of a circumference of an inner surface of die 1201. Wires 14(not shown in FIG. 12) travel through holes 1202 of die 1201 during thewire wrapping process. Die 1201 also comprises gap 1203 to control thespacing between respective groups of wires 14 applied to the corematerial 10 (not shown in FIG. 12). Also, each hole 1202 comprises anappropriate radii to preform its wire 14 as necessary for the wirewrapping process.

FIG. 13 depicts die 1301 for wire wrapping system 15 according to onerepresentative embodiment. As shown in FIG. 13, die 1301 also comprisesa plurality of features asymmetrically arranged about a circumference ofan inner surface of die 1301. In the case, the plurality of featurescomprises protrusions 1302 and larger protrusion 1303. Wires 14 (notshown in FIG. 13) proceed through the slots defined between adjacentones of protrusions 1302 and 1303. The size of protrusion 1303 controlsthe spacing between respective groups of wires 14 applied to the corematerial 10 (not shown in FIG. 13). The protrusions 1302 and 1303 may beappropriately shaped (e.g., possess appropriate radii) to preform wire14 as necessary for the wire wrapping process.

FIG. 14 depicts die 1401 for wire wrapping system 15 according to onerepresentative embodiment. Die 1401 includes a single feature 1402 overa limited arc of a circumference of an inner surface of die 1401.Feature 1402 is essentially a tab that extends above the remainingportion of die 1401. Feature 1402 separates wires 14 (not shown in FIG.14) to define the spacing between respective groups of wires 14 appliedto the core material 10 (not shown in FIG. 14). Wires 14 are otherwiseleft to find their own angular positions and are preformed as they passover the radii of die 1401 in a manner similar to conventional dies.

In one embodiment, a lead body is fabricated, in part, using wirewrapping system 15 such that the lead body is capable of elasticelongation under relatively low stretching forces. Also, after removalof the stretching force, the lead body is capable of resuming itsoriginal length and profile. For example, in one embodiment, relativelylow durometer, elastic polymer material is used for the material of thelead body. The combination of the selection of the materials, thehelically wrapping of the wires, and the repeating groups of wires withseparating gaps enables the stretching according to the relatively lowstretching forces. For example, the lead body may stretch 10%, 20%, 25%,35%, or even up to 50% at forces of about 0.5, 1.0, and/or 2.0 pounds ofstretching force. For additional description of a lead body capable ofelastic elongation, reference is made to U.S. Patent Publication No.2007/0282411, entitled “COMPLIANT ELECTRICAL STIMULATION LEADS ANDMETHODS OF FABRICATION,” which is incorporated herein by reference.

FIG. 15 depicts a cross-sectional view of lead body assembly 1500according to one representative embodiment. Lead body assembly 1500comprises stainless steel mandrel 1520 which is coated with layer 1501of PTFE. Inner layer 1502 of CARBOSIL™ is extruded or otherwise providedover the inner layer of PTFE. The mandrel 1520 with layers 1501 and 1502is utilized as core material 10 in wire wrapping system 15. Each wire1510 (only one wire is annotated for the sake of clarity) is preferablystranded wire coated with a thin layer of PFA and a thicker layer ofCARBOSIL™. Wire wrapping system 15 wraps a plurality of wires 1510 aboutmandrel 1520, layer 1501, and layer 1502 in the manner discussed above.An outer layer 1503 of CARBOSIL™ is also provided. Shrink wrap tubing1504 is then provided on the exterior of the assembly.

Lead body 1500 is cut to length and lead body assembly 1500 is subjectedto heating above the melting point of the thermoplastic material. Theheat and pressure (e.g., from heat shrinkable tubing) causes thethermoplastic insulative material (e.g., the CARBOSIL™ material) toflow. After the thermoplastic material is cooled, the thermoplasticmaterial re-solidifies into a lead body 1600 of fused insulativematerial enclosing the respective conductors 1510. Also, as shown inFIG. 16, gap 1610 is provided within lead body 1600 where no conductorsare located within gap 1610. That is, gap 1610 is entirely filled withinsulative material.

Lead body 1600 is then cut into appropriate lengths and electrodes andterminals are provided using any known or later developed process toform stimulation lead 1700 as shown in FIG. 17. Although lead 1700 isshown to fabricated as a “percutaneous lead,” other lead designs mayalso be employed such as paddle-style leads. Also, although someembodiments have discussed fabrication of neurostimulation leads, othermedical leads may be fabricated according to other embodiments, such ascardiac leads, mapping leads, ablation leads, etc.

Although certain representative embodiments and advantages have beendescribed in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the appended claims. Moreover, the scope of thepresent application is not intended to be limited to the particularembodiments of the process, machine, manufacture, composition of matter,means, methods and steps described in the specification. As one ofordinary skill in the art will readily appreciate when reading thepresent application, other processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the described embodiments maybe utilized. Accordingly, the appended claims are intended to includewithin their scope such processes, machines, manufacture, compositionsof matter, means, methods, or steps.

The invention claimed is
 1. A method of fabricating a stimulation leadfor electrical stimulation of tissue of a patient, the methodcomprising: drawing core material through a central portion of a die,wherein the die comprises one or more features asymmetrically arrangedabout a circumference of the die; rotating a plurality of payoutcarriers as a group while releasing conductor wires from the pluralityof payout carriers; rotating the die concurrently with rotation of theplurality of payout carriers; wrapping the released conductor wiresabout the core material to form a length of a lead body assembly,wherein the one or more features of the die direct the conductor wiresfrom the plurality of carriers onto the core material in an axiallyrepeating pattern of groups of closely spaced wires with each groupseparated by a distance larger than the spacing between adjacent wireswithin each group; forming a lead body using a portion of the length ofthe lead body assembly; and forming a stimulation lead from the leadbody; wherein the die is held by two or more support struts that aremechanically coupled to respective support columns; wherein the supportcolumns are coupled to a platform that is rotated at the same rate as aturntable that rotates the plurality of payout carriers.
 2. The methodof claim 1 wherein the one or more asymmetrically arranged featurescomprise a plurality of slots disposed about a limited portion of thecircumference of the die, the conductor wires being fed through theplurality of slots during the wrapping.
 3. The method of claim 1 whereinthe one or more asymmetrically arranged features comprise a plurality ofapertures disposed about a limited portion of the circumference of thedie, the conductor wires being fed through the plurality of aperturesduring operation of the wrapping.
 4. The method of claim 1 wherein theone or more asymmetrically arranged features comprise a plurality ofsurfaces disposed in respective planes that intersect at a central axisof the die, wherein the conductor wires contact the plurality ofsurfaces during operation of the wrapping.
 5. The method of claim 1wherein the platform and the turntable are mounted on a central cylinderstructure.
 6. The method of claim 1 wherein the platform comprises aplurality of recessed indentations, wherein the conductor wires aredrawn across the recessed indentations during operation of the system.7. The method of claim 1 wherein the conductor wires comprise strandedwire coated with one or more layers of insulative material.
 8. Themethod of claim 7 wherein the one or more layers of insulative materialcomprise an inner layer of perfluoroalkoxy and an outer layer of athermoplastic silicone polycarbonate urethane.